The prior art contains several suggestions of methods and apparatus for obtaining bi-axial orientation in thermoplastic material which has been formed into containers by the two-stage blow-molding technique. These prior art systems utilize "free-extrusion" blow molding machines wherein a pendant tube or parison of thermoplastic material is issued from a downwardly facing orifice. Two successive blowing operations are then performed on the parison by closing an initial preform mold on the parison and blowing and thereafter placing such preform into a second mold cavity for blowing to the final desired size and shape.
These two-stage blowing operations are particularly advantageous in the formation of large containers in that such technique does not require such a large parison, and therefore, substantially reduces the amount of flash and the accompanying requirement for reprocessing of the flash. Two-stage blowing also permits better and more uniform material distribution notwithstanding the increased size of the final product.
Heretofore, the sequential registration of the preform and final blow molds with the parison has been accomplished either by shuttling the molds over a stationary parison and preform or by transporting the preform from one mold to the other on a blow pipe. Examples of these prior art techniques are found in U.S. Pat. Nos. 3,767,747 and 3,781,395.